Nanoporous polymer foams

ABSTRACT

Process for the production of nanoporous polymer foams, comprising the stages
     a) loading of a single-phase, thermoplastic polymer melt with a gas under a pressure and at a temperature at which the gas is in the supercritical state,   b) heating of the laden polymer melt to a temperature which lies in the range from 40° C. under to 40° C. over the glass transition temperature of the unladen polymer melt determinable by means of DSC according to DIN-ISO 11357-2 at a heating rate of 20 K/min,   c) depressurization of the polymer melt loaded in stage a) and heated in stage b) with a depressurization rate in the range from 15,000 to 200,000 MPa/sec, and
 
the nanoporous polymer foams with a cell count in the range from 1,000 to 100,000 cells/mm and a density in the range from 10 to 500 kg/m 3  obtainable according to the process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of provisional application 61/346,913,filed May 21, 2010 which is incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to processes for the production ofnanoporous polymer foams by depressurization and nanoporous foamsobtainable thereby.

It is generally known that foamed plastics can be produced by extrusionof melts containing volatile propellants.

Thus in “Polymer Engineering and Science”, Vol. 38, No. 7, 1998, M. Leeet al. describe the extrusion of foamed polyethylene/polystyrene blendswith supercritical carbon dioxide.

Particularly in the thermal insulation field, foams are used asinsulating material. Since the mean free path of air is about 60 to 100nanometers (depending on pressure and temperature), it can be concludedfrom this that in a polymer foam with air as the cell gas at an averagecell size of less than or equal to 60 to 100 nanometers the contributionof the cell gas to the total thermal conduction of the foam issignificantly reduced or even completely eliminated. Hence foams with assmall-celled a structure as possible are especially desirable.

However it must be noted not only that the attainment of such a smallcell size is important, but also that the foam density must be reducedas far as possible, in order not to lose the advantage gained via thecell gas through an increased contribution of the polymer matrix to thetotal thermal conduction. This means that a nanoporous foam must alsohave as low a density as possible in order to have an improved thermalinsulating action compared to standard polymer foams.

In addition there is the problem that very small cell sizes can indeedoften be present directly after the foaming, but then a maturation takesplace with the formation of larger cells.

For example, in U.S. Pat. No. 5,955,511 and in EP-A 1 424 124 processesfor the production of micro- and nanoporous polymer foams are described,in which in a first step a polymer is loaded with a propellant underpressure at low temperatures below the glass transition temperature ofthe polymer. After depressurization without foaming, this laden polymeris then foamed in a separate step by increasing the temperature.

In WO2008/087559, continuous extrusion processes for the production ofnanoporous polymer foams are described, in which a polymer is admittedlyexposed to the propellant at different temperatures under pressure, butthe subsequent foaming process by depressurization is performed at verylow temperatures far below the glass transition temperature of the purepolymer but above the glass transition temperature of the gas-ladensystem.

In US2009/0130420, a continuous extrusion process for the production ofnanoporous polymer foams is described, in which a polymer melt is loadedwith a propellant under pressure and is foamed by subsequentdepressurization likewise in the region of the glass transitiontemperature of the gas-laden melt. Admittedly, high process pressures upto 1000 MPa are stated here for the loading, however the stateddepressurization rate of 10 to 1000 MPa/s in combination with the lowtemperatures once again leads to a comparatively high foam density.

However, not only do the processes described have process technologydisadvantages, but the product properties also reveal a need for furtheroptimization.

The systems produced are often microporous or macroporous. Here“microporous” means that the pore sizes lie in the range from 1 to 1000micrometers. The term “macroporous” designates dimensions greater than1000 micrometers.

Hence the purpose of the present invention is to find processes for theproduction of nanoporous polymer foams with improved applicationtechnology properties, which enable deliberate adjustment of the cellsize and the foam density with high and designated precision. Further,the processes should be simpler to perform than the known processes.

BRIEF SUMMARY OF THE INVENTION

Accordingly, a process for the production of nanoporous polymer foamswas found, comprising the stages

-   a) loading of a polymer melt formed from thermoplastic polymers with    a propellant under a pressure and at a temperature at which the    propellant is in the supercritical state,-   b) heating of the laden polymer melt to a temperature which lies in    the range from −40 to +40° C. around the glass transition    temperature of the pure polymer,-   c) depressurization of the polymer melt laden in stage a) is    effected at a depressurization rate in the range from 15,000 to    2,000,000 MPa/sec, and optionally-   d) comminuting the nanoporous polymer foam obtained in stage c) to    foam particles having an average particle diameter in the range from    10 μm to 10 mm.

A BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a scanning electron micrograph of the nanoporous foamfrom example 1.

FIG. 2 illustrates a scanning electron micrograph of the nanoporous foamfrom example 2.

FIG. 3 illustrates a scanning electron micrograph of the nanoporous foamfrom example 3.

FIG. 4 illustrates a scanning electron micrograph of the nanoporous foamfrom example 4.

FIG. 5 illustrates a scanning electron micrograph of the nanoporous foamfrom example 5.

FIG. 6 illustrates a scanning electron micrograph of the nanoporous foamfrom example 6.

FIG. 7 illustrates a scanning electron micrograph of the nanoporous foamfrom example 7.

FIG. 8 illustrates a scanning electron micrograph of the nanoporous foamfrom example 8.

FIG. 9 illustrates a scanning electron micrograph of the nanoporous foamfrom example 16.

FIG. 10 illustrates a scanning electron micrograph of the nanoporousfoam from example 10.

FIG. 11 illustrates a scanning electron micrograph of the nanoporousfoam from example 15.

FIG. 12 illustrates a scanning electron micrograph of the nanoporousfoam from example 17.

DETAILED DESCRIPTION OF THE INVENTION

Preferably, the polymer melt laden in stage a) is heated such that thetemperature at the instant of foaming lies in the range from −20 to +35°C. around the glass transition temperature of the unladen polymer melt.Especially preferred is a temperature range which deviates by about 0 to+30° C. from the glass transition temperature of the pure polymer.

The determinable glass transition temperature is described as the glasstransition temperature. The glass transition temperature can bedetermined by means of DSC according to DIN-ISO 11357-2 at a heatingrate of 20 K/min.

By means of the process according to the invention, nanoporous polymerfoams with an average cell count in the range from 1,000 to 100,000cells/mm, preferably from 2,000 to 50,000 and especially preferably from5,000 to 50,000 cells/mm, and a foam density in the range from 10 to 500kg/m³, preferably in the range from 10 to 300 kg/m³, especiallypreferably in the range from 10 to 250 kg/m³, are produced.

According to the invention, the term “nanoporous” comprises pore sizesin the range from 5 to 1000 nanometers.

According to the invention, the term “average cell size” describes theaverage diameter of spherical foam cells with cross-sectional areasequivalent to the real cells in typical frequency/size curves, such ascan be determined from evaluation of at least 10 real cell areas fromrepresentative electron micrographs.

According to the invention, the term “foam density” or also “density”describes the mass to volume ratio of the foamed nanoporous moldingcompound, which can be determined by the buoyancy method or is obtainedby calculation from the mass to volume quotient of a molded part.

According to the invention, the term “molding compound” or also “polymermelt” includes both pure homo- and also copolymers and mixtures ofpolymers. Furthermore, the term also includes formulations which arebased on polymers and a great variety of additives. For example, mentionmay be made here merely of process additives such as for examplestabilizers, flow aids, color additives, antioxidants and similaradditives well known to those skilled in the art.

The foams can be closed-cell, but are preferably open-cell.“Closed-cell” means that a discontinuous gas phase and a continuouspolymer phase are present.

“Open-cell” means that there is a bicontinuous system, in which the gasphase and the polymer phase are each continuous phases, the two phasesbeing interpenetrating phases.

The nanoporous systems have an open cell content (according to DIN-ISO4590) of more than 40%, preferably more than 50%, especially preferablymore than 75%. In the ideal case, at least 90% if not indeed practicallyall cells are open, i.e. the foam structure consists only of webs.

In the first stage (stage a)), a polymeric molding compound (polymermelt) is loaded with a gas or a fluid as propellant under a pressure andat a temperature at which the propellant is in the supercritical state.

As thermoplastic polymers for the polymer melts, for example styrenepolymers, polyamides (PA), polyolefins, such as polypropylene (PP),polyethylene (PE) or polyethylene-propylene copolymers, polyacrylates,such as polymethyl methacrylate (PMMA), polycarbonate (PC), polyesters,such as polyethylene terephthalate (PET) or polybutylene terephthalate(PBT), polysulfones, polyether sulfones (PES), polyether ketones,polyether imides or polyether sulfides (PES), polyphenylene ether (PPE)or mixtures thereof can be used. Especially preferably, styrenepolymers, such as polystyrene or styrene-acrylonitrile copolymers orpolyacrylates such as polymethyl methacrylate are used.

Thermoplastically workable amorphous polymers, in which not more than 3%of crystalline components are present (determined by DSC) areparticularly suitable as polymers.

Solid, gaseous or liquid propellants such as carbon dioxide, nitrogen,air, noble gases such as for example helium or argon, aliphatichydrocarbons such as propane, butane, partially or completelyhalogenated aliphatic hydrocarbons, such as (hydro)fluorocarbons,(hydro)chlorofluorocarbons, difluoroethane, aliphatic alcohols ordinitrogen oxide (laughing gas) are suitable as propellants, carbondioxide, laughing gas and/or nitrogen being preferred. Carbon dioxide isquite especially preferred.

According to the invention, this means that the propellant can bedispensed and/or injected directly supercritically, or the processparameters of the polymer to be injected at the time of injection lie ina range such that the propellant becomes supercritical under theseconditions. For CO₂ for example, the critical point lies at about 31° C.and 7.375 MPa; for N₂O for example the critical point lies at about36.4° C. and 7.245 MPa.

The propellant loading of the polymeric molding compound or melt canaccording to the invention be effected in a pressure chamber, e.g. anautoclave, or in a tool cavity or in an extruder. According to theinvention, the exact temperature of the polymeric molding compound inthis stage is unimportant, although a temperature over the criticaltemperature of the propellant and above the glass transition temperatureof the polymeric molding compound is advantageous for this first loadingstep, since the uptake of the propellant via diffusion processes isaccelerated at temperatures above the glass transition temperature ofthe polymeric molding compound and hence shorter loading times arepossible.

According to the invention, for the loading a pressure above thecritical pressure of the propellant is set, preferably greater than 10MPa, especially preferably greater than 20 MPa. This loading pressure isimportant for the generation of as high as possible a gas concentrationin the polymeric molding compound, and in the context of the technicalpossibilities of present-day pressure vessels can be set at up to 200MPa.

In one version according to the invention, the loading is effected in anextruder. In an advantageously configured version, the temperature ofthe polymeric molding compound in the region of the propellant injectionis above the glass transition temperature of the molding compound, sothat the propellant can distribute and dissolve very well and rapidly inthe melt. The injection pressure during this is generally set higherthan the melt pressure in this region. In an especially advantageousimplementation, the injection pressure is set at a constant high valuevia a pressure maintaining valve. Thereby according to the invention apropellant mass flow which can amount to 1 to 60 wt. %, in particular 5to 50 wt. % based on the mass flow of the polymeric molding compound isset. The upper limit for the propellant loading here is the saturationconcentration attainable with the parameters pressure and temperature ofthe laden melt before the nozzle, which can either be determinedempirically in the process or by gravimetric methods.

In a second stage (stage b)) of the processes according to theinvention, while maintaining the loading pressure greater than 10 mPa,preferably greater than 20 mPa, the laden molding compound is cooled toa temperature which lies between −40 and +40° C., preferably between −20and +35° C., especially preferably between 0 and 30° C. around the glasstransition temperature of the unladen polymeric molding compounddeterminable by DSC according to DIN-ISO 11357-2 at a heating rate of 20K/min.

In an implementation of the process according to the invention in theautoclave, this adjustment of the temperature of the polymeric moldingcompound can be effected after application of the loading pressure.Alternatively, this temperature can also already be set beforeapplication of the loading pressure. In both process versions, care mustbe taken to allow sufficient time for homogenization of the temperature,in particular after injection of the cold propellant into the cavity.Furthermore, in these process versions, care must be taken to allowsufficient time for attainment of the saturation concentration viadiffusion, in particular at high volumes of the polymeric moldingcompound.

In a further version according to the invention in the extruder, theladen molding compound is continuously cooled. For this, all equipmentknown to those skilled in the art from a cooling extruder up to mixersand coolers can be used in any number and combination. In order tomaintain the pressure in the laden molding compound, the use of meltpumps to increase the pressure, which can also be built into the processin any number and position, can be appropriate. An advantage of theimplementation according to the invention is also based on this in thatin fact a segmental buildup of the process line offers great controlover the local parameters pressure and temperature and rapid andhomogeneous cooling of the laden molding compound can be effected underpressure. It is however a condition that through a sufficient residencetime and thorough mixing a homogeneous distribution of the propellantmolecules is effected and the propellant can be completely dissolved inthe polymeric molding compound.

Surprisingly, the inventors' experimental studies have shown that,contrary to general expert opinion, a rapid depressurization of apolymeric molding compound laden and heated according to the inventionleads in the third stage (stage c)) to stable nanoporous polymer foamsof low density.

A third stage (stage c)) is a depressurization where the polymer meltladen with propellant in stage a) and heated in stage b) isdepressurized at a depressurization rate in the range from 15,000 to2,000,000 MPa/sec. The depressurization rate relates to the pressurejump which takes place within a period of one second prior to frothing.In this case, the pressure drop is at least 10 MPa.

The pressure before the depressurization can be determined via apressure sensor. Depressurization is usually to atmospheric pressure.However, a slight overpressure or underpressure can also be applied. Ingeneral, the reduction drop takes place jumplike within 0.1 to 10 ms.The depressurization rate can be determined, for example, by applying atangent in the region of the steepest pressure drop in thepressure-temperature diagram.

In the continuous embodiment involving the use of an extruder, thedepressurization rate is typically set via the shape of the nozzle. Anozzle which is generally used for this has at least one nozzle sectionwhich preferably has a length of 1 to 5 mm and a cross section of 0.1-25mm².

By setting a depressurization rate in the range from 15,000 to 2,000,000MPa/s, preferably in the range from 30,000 to 1,000,000 MPa/s,particularly preferably in the range from 40,000 to 500,000 MPa/s, apolymeric molding compound with very high propellant concentration andcorrespondingly low viscosity can be produced even with homogeneous foamtemperatures above the glass transition temperature of the non-ladenmolding compound to give a nanoporous foam morphology with at the sametime markedly lower foam density. It has been determined thatdepressurization rates of up to 200,000 MPa/s can be sufficient in somecases. In these cases, the process can be carried out in a simplifiedmanner.

According to the invention, this third stage (stage c)) can be effectedin different ways in the different process versions. In a version in theautoclave, the depressurization rate according to the invention can ifdesired be ensured via rapidly switching valves or via the controlledoperation of depressurization devices such as for example a burstingdisk. In a version according to the invention in a tool cavity, thedepressurization rate can be achieved through rapid expansion of thecavity.

In the preferred implementation according to the invention in anextruder, the depressurization rate is ensured by the discharge capacityof the extruder and the nozzle geometry.

The present invention further relates to other technically usableequipment and methods familiar to those skilled in the art for theproduction of such nanoporous polymer foams by the aforesaid rapiddepressurization according to the invention of a polymeric moldingcompound heated according to the invention.

Depending on the nozzle geometry used, particularly in the extrusionprocess, foam structures and finally polymer foams of various shapes canbe produced. In preferred implementations of the process according tothe invention, solid profiles, for example plates or else hollowprofiles are produced.

In an also preferred configuration of the process according to theinvention, in a further process step (optional stage d)) the polymerfoam is comminuted into molded bodies in the form of foamed polymerparticles, granules or powders, e.g. by means of a cutting disk, agranulator, a blade, a fly cutter or a mill. The comminution step herecan preferably be attached directly after the depressurization, but canalso be performed separately at a later time. During this, it can beadvantageous to cool the polymer foam, for example with ice-water, dryice or liquid nitrogen.

The comminution in stage d) can be effected in one or more stages, inthe latter case in one or more different apparatuses. For example, thenanoporous polymer foam can be subjected first to a precomminution andthen to a postcomminution. A postcomminution can advantageously beeffected in a cutting mill or a moving bed countercurrent jet mill. Theaverage diameter of the foam particles after comminution is preferablyin the range from 10 μm to 10 mm and more preferably in the range from0.1 to 1 mm.

Useful apparatuses for comminution include especially screw comminutors,rotary shears, single-shaft and multishaft comminutors, roll mills, finemills, pulverizers, impact disk mills, hammer mills and moving bedcountercurrent jet mills.

Processes and apparatuses for comminution of organic materials arewidely known to those skilled in the art. A person skilled in the artselects a suitable apparatus as a function of the amount to becomminuted, the desired throughput, the particle size to be achieved andthe brittleness of the material used.

The pulverulent or granular nanoporous polymer foams thus obtained, ormaterials comprising the pulverulent or granular nanoporous polymerfoams, can also be preferably used as thermal insulants.

Powder beds for insulation applications such as blow-in insulation forexample are already known to those skilled in the art. However, it hasbeen possible to date to process organic materials in the form of foamsinto powders and/or beds without adversely affecting the favorablethermal insulation properties of the underlying porous material. In thecase of known organic porous materials, the pore structure is completelydestroyed after comminution.

Known inorganic porous materials likewise do not permit a sufficientlyfree choice of the particle size, limiting employability. For example,fumed silicas as porous materials are generally generated as a finedust, so that pressing and/or adhesive bonding is required for manyapplications.

The flowability and low density of the nanoporous polymer foam both inthe loose bed and in the compacted state are of great advantage in lateruse. The particle diameters, which can be set to specific values in acontrolled manner, and also their size distribution through choice ofthe comminution process are a further advantage of loose beds.

The pulverulent or granular nanoporous polymer foams can be used assuch, or in mixture with further functional components, as thermalinsulants. A thermal insulant is accordingly an admixture comprising thepulverulent or granular nanoporous polymer foams. The selection ofsuitable functional components as added substances depends on the fieldof use.

The invention also relates to building materials and vacuum insulationpanels comprising the pulverulent or granular nanoporous polymer foamsand also to the use of the pulverulent or granular nanoporous polymerfoams for thermal insulation. The materials obtainable are preferablyused for thermal insulation particularly in built structures, or forcold insulation particularly in the mobile, logistical or stationarysector, for example in refrigerating equipment or for mobileapplications.

Possible further components for these thermal insulants are for examplecompounds capable of absorbing, scattering and/or reflecting thermalradiation in the infrared range, particularly in the wavelength rangebetween 3 and 10 μm. They are generally referred to as infraredopacifiers. The particle size of these particles is preferably in therange from 0.5 to 15 micrometers. Examples of substances of this kindare particularly titanium oxides, zirconium oxides, ilmenites, irontitanates, iron oxides, zirconium silicates, silicon carbide, manganeseoxides, graphites and carbon black.

Fibers can be used for mechanical reinforcement as added substances.These fibers can be of organic or inorganic origin. Examples ofinorganic fibers are preferably glass wool, rock wool, basalt fibers,slag wool, ceramic fibers consisting of melts of aluminum and/or silicondioxide and also further inorganic metal oxides, and purely silicondioxide fibers such as silica fibers for example. Organic fibers arepreferably cellulose fibers, textile fibers or polymeric fibers forexample. The following dimensions are used: diameter preferably 1-12micrometers and particularly 6-9 micrometers; length preferably 1-25 mmand particularly 3-10 mm.

Inorganic filling materials can be added to the mixture for technicaland economic reasons. It is preferable to use various, syntheticallyproduced polymorphs of silicon dioxide such as, for example,precipitated silicas, electric-arc silicas, SiO₂-containing fly dustsformed by oxidations of volatile silicon monoxide, in theelectrochemical production of silicon or ferrosilicon. Likewise silicasprepared by leaching of silicates such as calcium silicate, magnesiumsilicate and mixed silicates such as, for example, olivine (magnesiumiron silicate) with acids. Naturally occurring SiO₂-containing compoundssuch as diatomaceous earths and kieselguhr are also used. It is likewisepossible to use: thermally expanded minerals such as preferably perlitesand vermiculites. If required, preferably finely divided metal oxidessuch as preferably aluminum oxide, titanium dioxide and iron oxide canbe added.

The admixing of the thermal insulants can generally take place indiverse mixing assemblies. However, planetary mixers are preferablyused. It is advantageous here to first pre-mix the fibers with a portionof the second mixing component as a kind of masterbatch to therebyensure complete destructuralization of the fibers. After fiberdestructuralization, the largest portion of the mixing component isadded.

On completion of the mixing operation the bulk density of the mixturecan be between preferably 40-180 kg/m³ and more preferably 40-120 kg/m³,depending on component type and quantity. The flowability of theresulting porous mixture is very good, such that it can, inter alia,also be introduced and pressed, for example, into the cavities of hollowbricks without any problem and homogeneously, having been pressed intoplates. In the course of pressing into plates, by specifying particularplate thicknesses, via the weight, the density and consequently also thecoefficient of thermal conductivity of the insulant can be influencedsignificantly.

The materials used in thermal insulants are preferably used in thefollowing fields of application: as insulation in hollow bricks, as coreinsulation in multishell bricks, as core insulation for vacuuminsulation panels (VIPs), as core insulation for exterior insulationfinishing systems (EIFS), and as insulation in cavity walls,particularly in blow-in insulation.

The present invention further provides vacuum insulation panelscomprising the pulverulent or granular nanoporous polymer foams. Inaddition, the en thermal insulants and the pulverulent or granularnanoporous polymer foams are particularly useful for the insulation ofextruded hollow profiles, especially as core material for insulation inwindow frames.

Very good insulation performance is shown particularly by the so-calledvacuum insulation panels, VIPs for short. Having a thermal conductivityof about 0.004 to 0.008 W/mK (depending on core material and vacuum),vacuum insulation panels provide 8 to 25 times better thermal insulationperformance than conventional thermal insulation systems. Theyaccordingly allow slim-line structures with optimum thermal insulationperformance, which can be used not only in the building constructionsector but also in the household appliance, refrigeration and logisticssectors as well as in automotive or more general vehicle building.

Vacuum insulation panels based on porous thermal insulants, polyurethanefoam plates and pressed fibers as core material with composite foils(e.g., aluminum composite foils or so-called metalized films) are commongeneral knowledge and have been extensively described.

A further disadvantage of current vacuum insulation panels is themissing combination of low thermal conductivity at moderate pressuresand at low densities of the core materials below 200 kg/m³. The use ofthe nanoporous polymer foams as core thermal insulant in vacuuminsulation panels provides an optimum combination of thermalconductivity at low pressure, durability and low density as a functionof the parameters of cell size and foam density and also the setparticle size and particle size distribution. When used as corematerials, the pulverulent or granular nanoporous polymer foams can beused directly as a loose bed or as a pressed molding.

EXAMPLES Inputs

-   PMMA 6N: Plexiglass 6N PMMA from Evonik Röhm GmbH with a glass    transition temperature of about 102° C. (measured by DSC to ISO    11357-2, heating rate: 20K/min)-   PMMA 5N Plexiglass 5N PMMA from Evonik Röhm GmbH with a glass    transition temperature of about 98° C. (measured by DSC to ISO    11357-2, heating rate: 20K/min)-   PS156F Empera 156F polystyrene from Ineos Styrencis International SA    with a glass transition temperature of 102° C. (measured by DSC to    ISO 11357-2, heating rate: 20K/min)

Examples 1-15 Autoclave

For the following examples the polymers listed in Table 1 were used.After predrying of the granules for 3 hrs at 80° C. in the vacuum oven,about 200 mg of polymer in the form of granules in an in-houseconstructed, heatable vertical steel autoclave with an internal volumeof about 2.5 ml were brought to the foam temperatures stated in therelevant example. At the upper end, this autoclave is equipped with apressure sensor which measures the internal pressure at a rate of 1/ms.Pressure and temperature were continuously recorded via a computer andcould then be evaluated.

Here it should be noted that the foam temperature was taken as thedirectly measured temperature of the bursting disk located below, onwhich the polymer lay. By means of an automatic motor-driven pump (SITECmodel C) the relevant propellant in the supercritical state was thenpumped in and the relevant loading pressure applied. To compensate fortemperature fluctuations, the pressure was readjusted within the first 2hours until a stable equilibrium state and a stable temperature of thebursting disk had been established.

In order to ensure sufficient time for the uptake of the propellant viadiffusion processes, the sample was saturated for about 15 to 24 hrsunder constant conditions, even when an equilibrium state was alreadyestablished after a shorter time.

For the foaming of the molding compound laden and heated, the pressureof the supercritical propellant in the chamber was then increased viathe motor-driven pump over a period of a few seconds until attainment ofthe failure pressure of the bursting disk. The depressurization rate wasthen determined by evaluation of the pressure data of the sensor. Inthis, a linear pressure drop was assumed. All the foaming experimentsshowed an almost complete pressure drop in the region of 2 ms, while thefall below the saturation pressure decisive for the cell nucleation tookplace even more rapidly.

After emergence from the pressure chamber through the hole forming inthe bursting disk, the foamed sample was captured in a water-soakedsponge ca. 50 cm below the original position and could be stably handledand examined directly after the foaming process.

The density of the foamed molded bodies was determined by the buoyancymethod, while the cell parameters such as the average cell diameter weredetermined by evaluation of scanning electron micrographs of at least 2places in the foam. For the statistical evaluation, pictures with atleast 10 whole cells in the picture detail were used.

In examples 1-15, an optically homogeneous, translucent nanoporous foamwith an average density in the range from 100-300 kg/m³ and an averagecell diameter in the range from 40-210 nm was obtained. The polymerfoams had a markedly bluish appearance under incident light, andappeared slightly reddish to transparent in transmitted light.

Scanning electron micrographs are shown in FIGS. 1-11.

TABLE 1 Process parameters: Bursting Saturation disk Failure Pressurepressure temperature Saturation pressure drop rate Example PolymerPropellant [bar] [° C.] time [hr] [bar] [GPa/s] 1 PMMA 6N CO2 777 122 22956 47 2 PMMA 6N CO2 758 109 22 847 41 3 PMMA 6N CO2 762 95 22 937 46 4PMMA 6N CO2 500 127 22 752 38 5 PMMA 6N CO2 400 101 21 803 40 6 PMMA 6NCO2 400 87 22 804 26 7 PMMA 6N CO2 356 130 22 429 17 8 PMMA 6N N2O 345130 22 429 18 9 PMMA 6N N2O 707 130 22 827 41 10 PMMA 5N CO2 750 85 22850 42 11 PMMA 5N CO2 735 105 22 923 46 12 PMMA 5N CO2 750 123 22 897 4513 PS 156F CO2 600 129 15 750 37 14 PS 156F CO2 700 128 19 769 38 15 PS156F CO2 750 129 18 805 40

TABLE 2 Average density and average cell diameter of the nanoporousfoams of examples 1-15 Average density Average cell diameter Example[kg/m³] [nm] Figure 1 150 100 1 2 180 85 2 3 230 40 3 4 195 120 4 5 220100 5 6 300 70 6 7 230 150 7 8 125 140 8 9 150 85 10 270 80 10 11 214100 12 160 130 13 220 210 14 192 180 15 189 150 11

Example 16 Extruder

In a preferred implementation according to the invention, a nanoporouspolymer foam of low density was produced in a continuous extrusionprocess.

In this, a Plexiglass 6N PMMA from Evonik Röhm GmbH as supplied was usedas the polymeric molding compound. In stage 1, the polymeric moldingcompound was melted and homogenized in an extruder (Leistritz 18 mm) ata flow rate of 2.26 kg/hr. Directly after the plasticization of thepolymeric molding compound, supercritical CO2 at a pressure of about 475bar was injected into the molding compound at a melt temperature of ca.220° C. For this, a mass flow rate of about 0.780 kg/h CO2 was set,which results in a loading of about 34.5 wt. % based on the mass ofpolymer.

The laden molding compound was then lowered to a temperature of ca. 103°C. before the nozzle by means of mixing and cooling elements. Thepressure along the process line after the propellant injection was keptabove a minimum value of 350 bar by the use of melt pumps.

By extrusion of the laden molding compound under this pressure and atthis overall mass flow rate through a round nozzle of 0.5 mm diameterand a length of 1.8 mm, a depressurization rate according to theinvention of the polymeric molding compound heated according to theinvention in the region of 80,000 MPa/s could be set.

In this process, a continuously extruded, optically homogeneous,translucent nanoporous polymer foam with an average foam density ofabout 200 kg/m³ and an average cell diameter of about 100 nm wasobtained. The polymer foam had a bluish appearance under incident light,and appeared slightly reddish in transmitted light.

A scanning electron micrograph of the nanoporous foam from example 16 isshown in FIG. 9.

Example 17 Extruder

In a further preferred implementation according to the invention, ananoporous polymer foam of low density was produced in a continuousextrusion process.

In this, a Plexiglass 6N PMMA from Evonik Röhm GmbH as supplied was usedas the polymeric molding compound. In stage 1, the polymeric moldingcompound was melted and homogenized in an extruder (Leistritz 18 mm) ata flow rate of 1.8 kg/h. Directly after the plasticization of thepolymeric molding compound, supercritical CO2 at a pressure of about 470bar was injected into the molding compound at a melt temperature ofabout 220° C. For this, a mass flow rate of about 0.745 kg/h CO2 wasset, which results in a loading of about 41.4 wt. % based on the mass ofpolymer.

The laden molding compound was then lowered to a temperature of about100° C. before the nozzle by means of mixing and cooling elements. Thepressure along the process line after the propellant injection was keptabove a minimum value of 375 bar by the use of melt pumps.

By extrusion of the laden molding compound under this pressure and atthis overall mass flow rate through a round nozzle of 0.3 mm diameterand a length of 1.57 mm, a depressurization rate in the region of270,000 MPa/s of the heated polymeric molding compound could be set.

In this process according to the invention, a continuously extruded,optically homogeneous, translucent nanoporous polymer foam with anaverage foam density of about 180 kg/m³ and an average cell diameter ofabout 90 nm was obtained. The polymer foam had a bluish appearance underincident light, and appeared slightly reddish in transmitted light.

A scanning electron micrograph of the nanoporous foam from example 17 isshown in FIG. 12.

1. A process for the production of a nanoporous polymer foams,comprising the stages a) loading of a polymer melt formed fromthermoplastic polymers with a propellant under a pressure and at atemperature at which the propellant is in the supercritical state, b)heating of the laden polymer melt to a temperature which lies in therange from 40° C. under to 40° C. over the glass transition temperatureof the unladen polymer melt determinable by DSC according to DIN-ISO11357-2 at a heating rate of 20 K/min, c) depressurization of thepolymer melt laden in stage a) and heated in stage b) with adepressurization rate in the range from 15,000 to 2,000,000 MPa/sec. 2.The process according to claim 1, wherein the loading and the heating ofthe polymer melt are performed continuously in an extruder and thedepressurization is effected via a nozzle.
 3. The process according toclaim 1 or 2, wherein polystyrene, polymethyl methacrylate (PMMA),polycarbonate, styrene-acrylonitrile copolymers, polysulfones, polyethersulfone, polyether imide or mixtures thereof are used as thethermoplastic polymer.
 4. The process according to claim 3, wherein theladen polymer melt is heated in stage b) to a temperature in the rangefrom 50 to 250° C.
 5. The process according to one of claims 1 to 3,wherein the pressure lies in the range from 20 to 200 MPa in step a) andin the range from 0.01 to 1 mPa (absolute) after the depressurization.6. The process according to one of claims 1 to 5, wherein carbon dioxide(CO₂) or dinitrogen oxide (N₂O) is used as the gas.
 7. The processaccording to one of claims 1 to 6, wherein it additionally comprises astage d) comminuting the nanoporous polymer foam obtained in stage c) tofoam particles having an average particle diameter in the range from 10μm to 10 mm.
 8. A nanoporous polymer foam obtainable by the processaccording to one of claims 1 to 7, wherein the cell count lies in therange from 1,000 to 100,000 cells/mm and the density in the range from10 to 500 kg/m³.
 9. The use of the nanoporous polymer foam according toclaim 8 for thermal insulation.
 10. The use of the nanoporous polymerfoam according to claim 8 as core material in vacuum insulation panels.